There has been known an oil separator that separates oil mist contained in target gas from the gas. For example, an oil separator described in Patent Literature 1 uses a rotor provided between a gas inlet port and a gas discharge port to separate oil mist from the gas using centrifugal force. This rotor is structured with a plurality of separation disks placed one over the other. These separation disks are configured with truncated cone shape plate members that have the outer peripheral side parts curved obliquely upward such that the radius on the upper side becomes large. The inner circumferential side parts of the separation disks have formed thereto openings that penetrate the separation disks in the thickness direction. In this way, a space is formed to the inner circumferential side part of this rotor.
This oil separator introduces crankcase gas (blow-by gas) being the target gas into the space on the inner circumferential side of the rotor. The crankcase gas introduced into this space flows to the outer peripheral side of the rotor through the gaps created between the separation disks rotating at high speed and condenses the oil mist within these gaps to separate the oil mist from the crankcase gas.